A. Field of the Invention
This invention relates generally to defibrillator circuits, and more particularly to portable, automatic defibrillators that may be operated from an internal battery when no A.C. power source is available, and which may be operated directly from the A.C. power source even though the internal battery may be discharged.
B. Description of the Prior Art
Defibrillator circuits for applying an electrical shock to a patient undergoing the rapid irregular contractions of the muscle fibers of the heart known as fibrillation. The prior art devices used to administer such an electrical shock typically consisted of a storage capacitor that was charged to a relatively high potential by a D.C. power supply and discharged into the patient through the series combination of an inductor and a relay.
The early prior art devices were designed to operate from an A.C. power source, and the amount of charge accumulated on the capacitor prior to discharge was adjusted by adjusting the output voltage of the power supply by means of a variable voltage transformer or the like. The amount of charge accumulated by the storage capacitor was determined by measuring the voltage across the capacitor by means of a voltmeter or the like and adjusting the variable voltage transformer until the desired reading is obtained. The capacitor was then discharged into the patient by closing the relay contacts.
Other prior art defibrillators were designed to be used as portable units in an ambulance or in other environments where a source of A.C. power was not available. These units typically utilized a battery such as a rechargable nickel cadmium battery as the power source and incorporated a small line current powered power source for charging the battery. Both the line powered units and the battery powered units had provisions for connecting the defibrillator electrodes to a preamplifier so that an electrocardiogram could be taken utilizing the defibrillator electrodes.
While these units did provide a way to defibrillate a stricken patient, the units were relatively cumbersome and difficult to operate, and could be dangerous if improperly used. Furthermore, the A.C. line powered units could not be used when a source of alternating current was unavailable, and the battery operated units could not be used if the battery was discharged, since, in order to achieve lightness and portability, the power supply in the battery power units was designed only as a battery charging system and did not have sufficient capacity to operate the unit without the aid of the battery. All of these units required large amounts of power to operate them. Moreover, when the electrodes of the defibrillator were also used as sensing electrodes for the electrocardiogram, a separate relay or other isolation device, such as a resistor had to be placed between the electrocardiogram unit and the defibrillator electrodes to prevent the defibrillator discharge from damaging the electrocardiogram unit. The use of a separate relay increased the complexity of the unit and damage to the electrocardiogram unit could occur should the isolating relay fail. The use of a high impedance coupling between the electrocardiogram unit and the defibrallator electrodes attenuated the electrocardiogram signal and caused noise and interference to be picked up by the electrocardiogram. Consequently, shielded cables had to be used to connect the electrocardiogram units to the electrodes to reduce the noise pickup; however, the capacitance of the cable increased the leakage current between the chassis and the electrodes.
Finally, the prior art units had no way of automatically charging the storage capacitor to a preselected watt second level, and were inefficient in the use of power. The inefficiency became a particular problem in battery powered units.